Implantable and insertable medical devices are well known in the medical community. Many of these devices are configured to expand upon implantation or insertion into the body. For instance, angioplasty procedures are well known, in which a catheter is navigated through a lumen of a vertebrate subject to a site needing expansion. For example, a distal portion of a catheter containing a deflated balloon may be directed to an area of an artery that is substantially blocked, and that may be enlarged upon expansion of the balloon, typically by a hydraulic or pneumatic mechanism.
U.S. Patent Appln. Pub. No. 2004/0138733, the entire disclosure of which is hereby incorporated by reference, describes medical devices, which include the use of nanopaper for mechanical actuation. The medical devices may be provided, for example, in the form of a balloon catheter, in which the nanopaper is mounted about an electrode and into which an electrically conductive solution is dispersed. Actuation of the electrode causes generation of bubbles, which in turn causes the nanopaper, and thus the medical device to which it is applied, to expand. Whereas inner bubbles are generally trapped and act to expand the nanopaper, an issue encountered with devices of this type is that bubbles created at the outer surfaces may escape into the surrounding media. Without wishing to be bound by theory, it is believed that, due to the very high surface area of the carbon nanotubes, bubbles can arise at many locations throughout the nanopaper. Small bubbles have tremendous inner pressure. Normally when they contact one another, smaller bubbles merge to form larger ones as the pressure is decreased inside the larger bubbles. However in the case of nanopaper, bubbles cannot merge together due to the network of carbon nanotubes. They can only merge if they crack open the carbon nantube paper. The bubbles on the surface of the paper, however, don't have the restriction of being surrounded by carbon nanotubes and can readily merge together to become larger. If they get large enough, their upward force (due to gravity) eventually becomes larger then the adhesion force which keeps them sticking to the paper surface, and the bubbles depart from the surface.
Even if the actuator is sheathed, it is desirable to prevent bubbles from escaping from the surface as they and expand the sheath. For example, if the sheath happened to burst, this would allow the gas between the sheath and the nanopaper to escape into the body. If this occurs at high pressure, the gas bubbles will expand due to a drop in pressure and may cause a blockage in the arteries.
Other implantable and insertable medical devices are adapted to achieve enhanced or suppressed interactions with surrounding cells and tissue. For example, carbon nanotube materials have been shown to be an ideal matrix for endothelial cell growth. See, e.g., “Carbon Nanotube Bucky Paper Scaffold for Retinal Cell Transplantation,” NASA Ames Research Center, including spatial organization. This is likely due, at least in part, to the nanostructure and porosity of such materials. For example, it is known that nanostructured surfaces may directly interact with cell receptors, thereby controlling the adhesion or non-adhesion of cells to the surface. It is also noted that carbon nanotube materials are porous and therefore may allow for the flow of therapeutic agents, including growth factors and nutrients.
However, the mechanical robustness of many particle-based materials, including paper formed from carbon nanotubes, is in need of enhancement.